HAHAHA, oh you little noob, here, let me put it in terms that YOU can understand.
Bethesda did both Fallout and Oblivion.
both have Game of The Year editions that include DLC on a seperate disc. For fallout, its EVERY expansion pack. For Oblivion, its a GIGANTIC expansion pack and a "plug-in". I highly recommend Oblivion, because I felt it gave someone more to do, more to explore.
"memory unit" as you put it, is also known as a "hard drive", located on the top of the Xbox 360 console. It comes in 60 GB and 120 GB flavors. Downloading of the extra content is fast and easy, just put in the second disc, select to download ALL of the DownLoadable Content (DLC), and wait. it could take some time, so be PATIENT. I do not recomment borderlands or Civ Rev, cuz they're too iffy. many mixed reviews from both, you may hate or love either.
but Oblivion, I just beat the DLC, and it is BEAST. OBLIVION FTW. GET IT. NOW. HURRY UP. DO NOT PASS GO, DO NOT COLLECT 200$
oh yea, and if you wanted Fallout, dont buy the regular game, and then download the other packs, but Game Of The Year, cuz it costs a lot less then buying the retail disc, and then downloading all the packs seperately. ENJOY YOUR GAMING!
This has been a ChillzMaster N00B assistance.